What Screens are Made of: New Twists (and Bends) in LCD Research

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Researchers found that the spiral “pitch,” or width of one complete spiral turn, becomes a little longer with increasing temperature, and the spiral abruptly disappears at sufficiently high temperature as the material adopts a different configuration.

“Currently, this experiment can’t be done anywhere else,” Zhu said. “We are the first team to use this soft X-ray scattering technique to study this liquid-crystal phase.”

Standard LCDs often use nematic liquid crystals, a phase of liquid crystals that naturally align in the same direction—like a group of compass needles that are parallel to one another, pointing in one direction.

In these standard LCD devices, rod-like liquid crystal molecules are sandwiched between specially treated plates of glass that cause the molecules to “lie down” rather than point toward the glass. The glass is typically treated to induce a 90-degree twist in the molecular arrangement, so that the molecules closest to one glass plate are perpendicular to those closest to the other glass plate.

It’s like a series of compass needles made to face north at the top, smoothly reorienting to the northeast in the middle, and pointing east at the bottom. This molecularly twisted state is then electrically distorted to allow polarized light to pass through at varying brightness, for example, or to block light (by straightening the twist completely).

Future experiments will explore how the spirals depend on molecular shape and respond to variations in temperature, electric field, ultraviolet light, and stress, Zhu added.

He also hopes to explore similar spiraling structures, such as a liquid crystal phase known as the helical nanofilament, which shows promise for solar energy applications. Studies of DNA, synthetic proteins, and amyloid fibrils such as those associated with Alzheimer’s disease, might help explain the role of handedness in how organic molecules self-assemble.

With brighter, more laser-like X-ray sources and faster X-ray detectors, it may be possible to see details in how the spiraling twist-bend structure forms and fluctuates in real time in materials, Zhu also said.

“I am hoping our ongoing experiments can provide unique information to benefit other theories and experiments in this field,” he noted.

Other team members include Anthony Young, Cheng Wang, and Alexander Hexemer at Berkeley Lab, and Michael Tuchband, Min Shuai, Alyssa Scarbrough, David Walba, Joseph Maclennan, and Noel Clark at the University of Colorado Boulder.

Soft X-ray scattering measurements were conducted at Beamline 11.0.1 at the Advanced Light Source, a DOE Office of Science User Facility at Berkeley Lab. The work was supported by the DOE Office of Basic Energy Sciences and the National Science Foundation.

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